Phil Wainewright provides a reminder of what all these nines mean:

  • 99.5% — 43.76 hours (an entire working week, and more)
  • 99.7% — 26.30 hours (more than three working days)
  • 99.9% —  8.77 hours (more than one working day)
  • 99.95% — 4.38 hours (half a working day)
  • 99.97% — 2.63 hours (an extended lunch break)
  • 99.99% — 0.88 hours (about 50 minutes)
  • 99.995% — 0.44 hours (a half hour)
  • 99.999% — 0.09 hours (five minutes)

So 'the 5 nines' = 5 minutes down in a year.